1. Field of the Invention
This invention relates to the growth of single-crystal magnetoplumbite in either bulk form or expitaxial form.
2. References
Nielsen et al., "The Growth of Single Crystals of Magnetic Garnets," J. Phys. Chem. Solids, 1958, Pergamon Press, Vol. 5, pp 202-207.
Blank et al., "The Growth of Magnetic Garnets by Liquid Phase Epitaxy," Journal of Crystal Growth, 1972, North-Holland Publishing Co., Vol. 17, pp 302-311.
Jonker, "Investigation of the Phase Diagram of the System PbO-B.sub.2 O.sub.3 -Fe.sub.2 O.sub.3 -Y.sub.2 O.sub.3 for the Growth of Single Crystals of Y.sub.3 Fe.sub.5 O.sub.12," Journal of Crystal Growth, 1975, North-Holland Publishing Co., Vol. 28, pp. 231-239.
Mountvala et al., "Phase Relations and Structures in the system PbO-Fe.sub.2 O.sub.3," Journal of the American Ceramic Society, 1962, Vol. 45, No. 6, pp. 285-288.
All of the above-listed references are hereby incorporated by reference into this specification in their entirety.
3. Description of the Prior Art
As used herein, the term magnetoplumbite refers to the mineral PbO-6Fe.sub.2 O.sub.3 and to other compounds having the same crystal structure wherein other chemical elements replace some of the iron, some of the lead or some of both the iron and lead.
Magnetoplumbite is a ferrite useful in the fabrication of electronic devices such as millimeter-wave filters and resonators. In such devices, the ferrite is required to be in single-crystal form, either bulk single crystals or in the form of epitaxial single-crystal layers grown on appropriate single-crystal substrates.
In Nielsen et al., it is shown that bulk single crystals of magnetoplumbite can be grown at relatively high temperatures from a fluxed melt consisting of lead oxide (PbO), iron oxide (Fe.sub.2 O.sub.3) and a rare-earth oxide or yttrium oxide. Nielsen et al. used the crystal growth method known as "slow cooling." Their fluxed melts were held at a high temperature of from 1300.degree. C. to 1370.degree. C. for a few hours and then were slowly cooled at rates of from one to five degrees Celsius per hour until the desired final temperatures were reached. In some cases, the final temperature was less than 1000.degree. C. However, in all cases, crystallization initiated at temperatures much higher than 1000.degree. C. and all, or very nearly all, crystal growth occurred at temperatures above 1000.degree. C. Moreover, in all cases in which magnetoplumbite crystals were obtained, YIG also crystallized. Thus, magnetoplumbite was not obtained as a single phase. Where magnetoplumbite alone is desired, further processing is required to separate the phases.
In their FIG. 7, Nielsen et al. presented a partial phase diagram suggested for the system PbO-Y.sub.2 O.sub.3 -Fe.sub.2 O.sub.3 which shows a range of fluxed melt compositions from which, it is suggested, magnetoplumbite can be grown. They indicate that, because of the small amount of data obtained by them and other uncertainties in their determination, their phase diagram should be considered only "semi-quantitative."
In Blank et al., it is intimated that the diagram of Nielsen et al. is valid, at least in its general features, for PbO-B.sub.2 O.sub.3 fluxes. Blank et al. were interested in growing magnetic garnets by liquid phase epitaxy, which primarily entails crystallization at temperatures below 1000.degree. C. The Blank et al. version of the Nielsen et al. diagram includes a line labeled 1300.degree. C. and another line labeled 950.degree. C. The Blank et al. diagram hints at the possibility of forming single-crystal magnetoplumbite at temperatures between these two values. However, there is no discussion or description in the Blank et al. paper of these features of their phase diagram, nor is any data on magnetoplumbite crystallization presented. In fact, Blank et al. reported "no evidence of magnetoplumbite" even when the rare earth oxide (Y.sub.2 O.sub.3) concentration in their melt was very low. Thus the lines marked 1300.degree. C. and 950.degree. C. appear to have been intended to apply only to crystallization of the garnet phase. Blank et al. imply that adding boron oxide to the fluxed melt of Nielsen et al. does not significantly change the conditions under which magnetoplumbite forms.
Jonker reported on a more detailed investigation of the PbO-B.sub.2 O.sub.3 -Fe.sub.2 O.sub.3 -Y.sub.2 O.sub.3 system for the growth of single-crystal YIG at temperatures in excess of 1000.degree. C. He studied fluxes of three different compositions distinguished by changing their B.sub.2 O.sub.3 :PbO molar ratios from 0.0:1 to approximately 0.0:1 to approximately 0.2:1. Included in Jonker's study were a few data points for compositions which contained no yttrium oxide (Y.sub.2 O.sub.3). Jonker's crystal growth method was essentially the same slow cooling method as was used by Nielsen et al. as discussed above. Jonker used two different temperature ranges. One range started at 1275.degree. C. with slow cooling to a final temperature of 1000.degree. C. Jonker shows phase diagrams having lines labeled 1000.degree. C. and other lines labeled 1100.degree. C. These labels refer to the final temperatures of crystal growth runs. The temperature at which crystallization initiated must have been much higher. A third line in Jonkers phase diagrams is labeled 1200.degree. C. This line is only an estimate.
In addition to the crystal growth date presented, Jonker reports differential thermal analysis (DTA) data. DTA was carried out on samples of the PbO-B.sub.2 O.sub.3 fluxes containing Fe.sub.2 O.sub.3 in concentrations ranging from zero to 50 mole percent in steps of ten mole percent. The DTA samples did not contain any yttrium oxide. In presenting the DTA results, Jonker defines the composition range "relevant to bulk flux growth" as Fe.sub.2 O.sub.3 concentrations in the range from 20 to 50 mole percent. He further states that this composition range corresponds "to a single primary field, namely that of PbFe.sub.12 O.sub.19." However, Jonker does not indicate how or whether he determined that PbFe.sub.12 O.sub.19, equivalent to PbO.6Fe.sub.2 O.sub.3 magnetoplumbite, actually formed. The work of Mountvala et al., as discussed below, contradicts this conclusion by Jonker. Moreover, under the experimental conditions of DTA, phases are expected to crystallize in a ceramic or polycrystalline form rather than as single crystals.
If Jonker's DTA results could be applied to single-crystal growth, then it would appear that crystallization of magnetoplumbite at temperatures below 1000.degree. C. would be possible by using a melt having Fe.sub.2 O.sub.3 concentrations near 20 mole percent. For example, based upon Jonker's FIG. 1, using Fe.sub.2 O.sub.3 in a concentration of 20 mole percent and a lead oxide flux containing no boron oxide, the liquidus temperature would be 865.degree. C. Presumably, this is the temperature at which crystallization would initiate. This result is apparently correct; but the crystallizing phase at this temperature cannot be magnetoplumbite! In this connection, see Mountvala et al., referenced above. That the crystallizing phase under the conditions set forth above cannot be magnetoplumbite is shown by the phase diagram of Mountvala et al, for the PbO-Fe.sub.2 O.sub.3 system. The work of Mountvala et al., based on investigation of polycrystalline, ceramic materials, shows that there are three Pb-Fe-oxides. Besides the magnetoplumbite which Mountvala et al. denote as the .beta.-phase, there are .gamma.- and .delta.-phases.
The .delta.-phase has the composition 2PbO.Fe.sub.2 O.sub.3 ; the .gamma.-phase is PbO.2Fe.sub.2 O.sub.3 with a capacity to form solid solutions up to PbO.21/2 Fe.sub.2 O.sub.3 ; the magnetoplumbite .beta.-phase is PbO.6Fe.sub.2 O.sub.3 with a capacity to form solid solutions up to PbO.5Fe.sub.2 O.sub.3. It is clear from the phase diagram of Mountvala et al. that crystallization from PbO-Fe.sub.2 O.sub.3 melts at temperatures below 910.degree. C. must result in the formation of .delta.-phase (or the formation of PbO if the Fe.sub.2 O.sub.3 concentration is less than about 18 mole percent). Similarly, in the temperature range from 910.degree. C. to 945.degree. C., .gamma.-phase will crystallize. If the work of Mountvala et al. is to be accepted, only at temperatures above 945.degree. C. can magnetoplumbite crystallize. Therefore, Jonker's statement that his DTA results for 20 mole percent Fe.sub.2 O.sub.3 in PbO correspond to crystallization of magnetoplumbite must be wrong. In addition, there must be serious doubts about the validity of Jonker's statement for the other data points at 20 mole percent Fe.sub.2 O.sub.3, especially for the B.sub.2 O.sub.3 :PbO molar ratio of 0.1 for which Jonker's liquidus temperature is 930.degree. C.
It should be noted, as well, that none of the single-crystal prior art references (Nielsen et al., Blank et al. and Jonker) mentions any of the Pb-Fe-oxide phases other than magnetoplumbite. The other phases, .gamma. and .delta., were omitted by them since these phases do not exist at the high temperatures, greater than 1000.degree. C., employed in their work. Similarly, PbO is not expected to crystallize at temperatures greater than 1000.degree. C.; so crystallization of PbO is not mentioned.
In summary, the prior art indicates that single crystals of magnetoplumbite can be grown by slow cooling from a PbO or PbO-B.sub.2 O.sub.3 fluxed melt at temperatures above 1000.degree. C. Nothing in the prior art indicates that you would get magnetoplumbite when the iron oxide component of the flux is reduced enough to get crystallization below 1000.degree. C. Moreover, the published phase diagram for the PbO-Fe.sub.2 O.sub.3 system indicates that magnetoplumbite will not crystallize from the melt at temperatures below 945.degree. C.
The prior art techniques for growing single-crystal magnetoplumbite from either a PbO flux or a PbO-B.sub.2 O.sub.3 flux at temperatures at or above 1000.degree. C. have severe limitations. Some of the limitations are due to the relatively high volatility of lead oxide. This creates problems since lead-oxide vapors are both toxic and corrosive. In addition, the considerable and continuous evaporation of lead oxide from the melt at these elevated temperatures results in continuous shifts of composition of the fluxed melt with concomitant shifts in crystal growth rate, saturation temperature, impurity incorporation and other parameters of the process. These shifts are generally undesirable since they make it difficult to control crystal growth.
In attempts to avoid the difficulties arising from the high volatility of lead oxide at the high temperatures used, the prior art has employed sealed crucibles to prevent the escape of lead oxide. However, sealed crucibles make it difficult to carry out seeded growth, especially for liquid phase epitaxy.
Furthermore, the use of high growth temperatures, above 1000.degree. C., entails a relatively large temperature excursion in cooling the system to room temperature and therefore entails larger thermal stresses which can damage the crystals. In the case of epitaxial growth, these stresses can be particularly troublesome. Also, in epitaxial growth and some seeded growth situations, high growth temperatures exacerbate attack of the substrate by vapors and by melt. The high temperatures may also exacerbate interdiffusion between the substrate and the epitaxial layer growing thereon.
The need to sustain high temperatures raises the cost of the crystal growing process since energy consumption is high. The furnaces and other components used must be able to withstand the high temperatures. This requirement makes the equipment relatively expensive.
For all of these reasons, there is a need to provide means and methods for growing single-crystal magnetoplumbite at temperatures below 1000.degree. C.